Composite articles containing two or more rigid layers are known. Examples of such articles include magnetic recording tapes and discs, abrasive laminates, flexible printing plates, printable label stock and the like. A major problem in satisfactorily joining two or more of such materials together is their inability to adhere adequately to one another.
A number of techniques have been employed to overcome this problem. Such techniques include subjecting one or more of the surfaces to be bonded to high energy, such as from corona discharge, electron beam discharge, flame treatment and the like. Alternatively, chemical treatments such as modifying the surfaces by chemical etching, and applying primer layers have been employed. Such techniques require the use of expensive equipment and may expose the operator to high energy radiation or dangerous chemicals. Furthermore, they have not proven to be entirely satisfactory.
Examples of chemical treatments include treating the surface with para-chlorophenol. However, this approach provides only minimal improvement in adhesion and suffers from the major disadvantage that para-chlorophenol is hazardous to human health.
Application of primers has also been less than satisfactory. Often the substrate surface must be treated in some way (e.g., subjected to high energy) in order that the primer will exhibit sufficient adhesion to it. Also, primers frequently must be compounded to meet the needs of a specific coating or class of such coatings.
Yet another approach is to incorporate aziridine-containing materials into functional coatings (e.g., reflective layers, antihalation layers, gelatin layers, etc.) which contain groups reactive with the aziridine functionality. These coatihgs are then applied to a substrate. See, for example, U.S. Pat. Nos. 3,017,280; 4,167,414; 4,181,528; and 4,241,169.
These references disclose that the substrate and the functional coatings must each contain groups reactive with the aziridine group so that the aziridine essentially acts as a tie layer to better anchor the functional coatings to the substrate through crosslinking reactions. They also generally disclose that substrates which do not inherently contain the requisite surface functionality to react with the aziridine (i.e., inert materials) must undergo prior modification by oxidative treatment to incorporate that functionality. Such substrates include, for example, conventional polyesters such as poly(ethylene terephthalate), conventional polyolefin films such as polyethylene, polypropylene and polystyrene.
U.S. Pat. No. 3,704,167 discloses the use of polyazirdine-functional materials to improve the adhesion of a subsequently applied magnetic recording layer to an antihalation layer on motion picture (i.e. cinema) film.
U.S. Pat. No. 4,418,164 discloses coating aramid filaments with polyfunctional aziridines. The coated filaments are used as reinforcing fibers in unsaturated polyester matrices wherein they are totally encased in the matrices.
Copending application Ser. No. 745,261, filed June 14, 1985 (Canty et al.), discloses the use of a composition consisting essentially of a compound having at least one aziridine functionality or group to render an essentially flat surface more adherant to subsequently applied layers. The present invention represents an improvement over that disclosure.